Leaf form and photosynthetic physiology of Dryopteris species distributed along light gradients in eastern North America

摘要

1 Despite the ubiquity of ferns and at least tacit recognition by botanists that their physiology is unique among land plants, most studies on fern physiology have focused on only a few, locally distributed, usually distantly related species. No previous study has attempted to examine physiological adaptations in a group of widespread taxa that are closely related and whose relationships are well understood. Here we report leaf form and physiological measures for such a group, the 11 eastern North American species of Dryopteris (Dryopteridaceae), and examine differences in these parameters for evidence of adaptation to light availability. Economic theory predicts that species from sunnier habitats should have narrower, more steeply inclined leaves, lower specific leaf area, higher stomatal density, higher rates of maximum photosynthesis, respiration and stomatal conductance, and require more light to saturate photosynthesis. Species should show adaptive crossover in net carbon uptake per unit leaf mass, with a relative advantage by sun-associated species at high photon flux densities (PFDs) and by shade-associated species at low PFDs. Field studies allow us to begin characterizing the range of native light environments occupied by members of this group, and to examine interspecific variation in several aspects of leaf form and photosynthetic light response for evidence of adaption to light availability. We also present a novel means for incorporating phylogeny in tests of correlated evolution in a reticulate lineage. Synthesis. Observed trends in physiology and morphology generally agree with qualitative predictions but are often not statistically significant. We found no support for adaptive crossover in mass-based carbon uptake, and thus for light availability being the most important variable driving morphological and physiological adaptation in these ferns. We propose that hydraulic factors related to water balance may have played a larger role in determining their morphological and physiological variation. Allopolyploid hybrids did not show transgression in any physiological parameter that may have allowed them to coexist regionally with their parents. The results of our phylogenetically structured analyses highlight the importance of incorporating phylogeny into comparative studies, particularly when hybrid or polyploid taxa are present.